1. Field of the Invention
This invention relates to rackets for playing sports such as tennis and more specifically to tennis rackets having metal frames.
2. Description of the Prior Art
While metal rackets have been the subject of many patents issued in the United States and elsewhere, for example, U.S. Pat. Nos. 1,524,826 (Icke et al), 1,548,134 (Gallaudet) and British Pat. No. 185,275 accepted Sept. 7, 1922, the acceptance of metal rackets by tennis enthusiasts and particularly professional tennis players has been very slow and only relatively recently have some of the professionals switched from wooden-framed to metal-framed rackets. The reasons are manifold but one of the most prominent reasons has been structural failure of the frame, usually in the region of high-level stressing during use of the racket.
Tubular steel frames were introduced with moderate success but they involved welded joints, such joints being subject to defects and possible breakage and making the racket undesirable for "pro" shop sales.
In my U.S. Pat. No. 3,702,189 I disclosed and described in detail one-piece metal tennis racket frames formed by casting and I suggested the advantages of forging a racket frame from aluminum for added strength. Since the time of my filing the application that resulted in that patent I have engaged in extensive developmental work to optimize the design of a forged aluminum frame.
The inter-related factors which had to be considered were: Weight, Wind Resistance, Strength, Torsional Stability, Balance, Appearance, Reproducibility in Production, Cost of Production, Life and Playability. The meanings of all those factors, except "playability" are quite apparent. The term "playability" is much less precise in its meaning because it includes many of the recited parameters, but it also includes at least one additional parameter which I had not considered in my first design and manufacture of forged aluminum frames. That factor became apparent soon after my first forged-frame rackets (and the first forged-frame rackets to be produced in volume by anyone to my knowledge) got into the hands of semi-professional and professional players. I began to get complaints of "chattering" or undesired vibrations in the handle when a ball struck the strings near the portion of the metal head most remote from the handle.
The cause of this undesirable phenomenon was not apparent but, after struggling with the problem for a period of time, I discovered that the cause of the "chattering" was that when a ball hit the strings near the portion of the rim or head furthest from the handle, instead of strings yielding the frame was yielding. Directional control of the ball was thus lost, and for a professional player that is a serious defect. Greater rigidity of the frame in that region was required. For a given alloy of aluminum the greater strength can only be achieved by a dimensional change, i.e., an increase in the crosssectional area in the affected region of the frame. However, such dimensional changes in one region cannot be made without making compensating changes elsewhere, for balance and strength must also be maintained in a tennis racket for playability and stability.